The present invention relates to a circuit arrangement for operating light emitting diodes and to a method for achieving this purpose.
Conventional light emitting diodes (LEDs) emit light within a limited spectral range. FIG. 1 shows, for example, spectra of a blue 1, green 2, yellow 3 and red 4 light emitting diode. Modules are known in which light emitting diodes of different colors, e.g. blue and yellow (two LEDs) or red, green and blue (RGB) are combined in such a way that their light is mixed, for example, by means of a diffusion screen and that the mixed light appears white or that the spectrum 5 of the light resulting therefrom extends over the whole visible range.
Although this light appears fundamentally “white” there are troughs 6, 7 within the spectrum of this emitted light. These troughs have a disadvantageous effect in that, for example, objects with colors in the range of these gaps are rendered with a very matt appearance. The quality of the color rendering, which is expressed using the color rendering index or CRI photometric variable, is accordingly dependent on these gaps.
The color rendering index expresses how close the color rendering of an artificial lighting means comes to the broadly distributed continuous spectrum of natural sunlight. As is generally known, this cannot be expressed solely by the color temperature because the color temperature does not indicate whether there may be gaps in the spectrum of an artificial lighting means.
These spectral gaps thus arise when RGB light emitting diodes are connected to each other. However, these troughs are also found when so-called white light emitting diodes are used. These are light emitting diodes which are combined with photoluminescent material (fluorescence stain, luminescent material). The light from the LED chip in a first spectrum is partially converted into a second spectrum by the phosphorous layer or color conversion layer formed thereby. The mixture of the first and second spectrum then produces the spectrum of white light.
FIG. 2 shows the spectrum of such a white light emitting diode. With the aid of a color conversion layer, shortwave light such as, for example, blue light 8 can be converted into longwave light, for example, in the yellow or red wavelength range 9.
However, between the actual (e.g. blue) spectrum 8 of the lighting means chip and the second (yellow or red) shifted spectrum 9 of the conversion layer there is also conventionally a spectral gap or at least a spectral trough 10 so that the quality of the color rendering or the color rendering index is reduced as a result. Thus, there is a need for improvements in the art for these type devices.